Fatigue testing device for wind turbine blade testing, a method of testing wind turbine blades and a control system for a blade testing actuator

Abstract

The invention provides a fatigue exciter for wind turbine blades. Wind turbine blades require excitation at or near their natural frequency to induce bending moments that simulate in service loadings and must be easily controllable and with the minimum of unnecessary added mass or force. The invention provides a device and a method by which force controlled feedback is used for finding an optimal excitation frequency. This force could be provided e.g. by a digital signal generator.

What is claimed is: 
     
       1. A fatigue testing device for a wind turbine blade, the blade having a hub end and a tip end and a longitudinal axis, the device comprising:
 a ground supported bench configured to have the hub end of the blade operably fixed thereto such that the blade is cantilevered therefrom, and 
 a ground supported actuator spaced from said bench and configured to be connected to the blade at a position between the hub end and the tip end of the blade, 
 said bench and said actuator configured to support the blade such that the longitudinal axis of the blade is in a generally horizontal attitude, 
 said actuator configured to reciprocatingly translate the blade at an excitation frequency in a direction generally perpendicular to the longitudinal axis of the blade and generally normal to the ground, said actuator configured to be attached to the blade in such a manner that the structural dynamics characteristics of the blade cantilevered from said bench and with said actuator attached thereto are substantially the same as they would be were the blade only cantilevered from said bench. 
 
     
     
       2. The fatigue testing device of  claim 1  further including a control system configured to find an initial natural frequency of the blade and to set the excitation frequency to the initial natural frequency, said control system further configured to find a subsequent changed natural frequency of the blade and to set the excitation frequency to the subsequent changed natural frequency. 
     
     
       3. The fatigue testing device of  claim 2  wherein said control system is configured to find the subsequent changed natural frequency of the blade by modulating the excitation frequency so that the excitation frequency is repeatedly swept forwards above and backwards below the initial natural frequency, and during modulation of the excitation frequency, monitoring blade amplitude and an input force of said actuator necessary to produce a desired blade amplitude, and if the necessary input force decreases during forward sweeping of the excitation frequency to continue forward sweeping of the excitation frequency, and if the necessary input force increases during forward sweeping of the excitation frequency to cause backward sweeping of the excitation frequency. 
     
     
       4. The fatigue testing device of  claim 3  wherein said control system is configured to modulate the excitation frequency so that the excitation frequency is continuously swept forwards above and backwards below the initial natural frequency of the blade. 
     
     
       5. The fatigue testing device of  claim 1  further comprising a first sensor configured to sense an input force to the blade from said actuator, a second sensor configured to sense an amplitude of deflection of the blade due to said actuator, and a control system in communication with said actuator and said first and second sensors, said control system including a controller. 
     
     
       6. The fatigue testing device of  claim 5  wherein said control system is configured to monitor the amplitude of the blade as sensed by said second sensor, to find an initial natural frequency of the blade based on the amplitude of the blade sensed by said second sensor, and to set the excitation frequency of said actuator to the initial natural frequency. 
     
     
       7. The fatigue testing device of  claim 6  wherein said control system further includes a frequency modulation algorithm and is further configured to modulate the excitation frequency so that the excitation frequency is repeatedly swept forwards above and backwards below the initial natural frequency. 
     
     
       8. The fatigue testing device of  claim 7  wherein said control system is configured to find a subsequent natural frequency to set the excitation frequency to by, during modulation of the excitation frequency, monitoring blade amplitude via said second sensor and the input force of said actuator necessary to produce a desired blade amplitude via said first sensor, and if the necessary input force decreases during forward sweeping of the excitation frequency, to continue forward sweeping of the excitation frequency, and if the necessary input force increases during forward sweeping of the excitation frequency, to cause backward sweeping of the excitation frequency. 
     
     
       9. The fatigue testing device of  claim 8  wherein the control system is further configured, during modulation of the excitation frequency, to provide an input signal significant of an increase or decrease of the input force necessary to deflect the blade and to provide an output signal significant of the amplitude of the deflection and further configured to select a subsequent excitation frequency based on the input signal, the output signal, or a ratio therebetween. 
     
     
       10. The fatigue testing device of  claim 8  wherein the control system is further configured, during modulation of the excitation frequency, to provide an input signal significant of an increase or decrease of the input force necessary to deflect the blade and to provide an output signal significant of the amplitude of the deflection and further configured to select a subsequent excitation frequency based on a ratio between the input force and the amplitude, the input signal, or the output signal, wherein the selection facilitates a minimal ratio between the input force and the amplitude. 
     
     
       11. The fatigue testing device of  claim 8  further comprising at least one strain sensitive structure being attachable to the blade, said device being further configured to modulate the excitation frequency based on a signal generated by said strain sensitive device. 
     
     
       12. The fatigue testing device of  claim 8  further comprising an additional actuator configured to deflect the blade at an additional excitation frequency. 
     
     
       13. The fatigue testing device of  claim 12  wherein the additional excitation frequency is adjustable. 
     
     
       14. The fatigue testing device of  claim 12  wherein the additional excitation frequency is adjustable independent of the excitation frequency. 
     
     
       15. The fatigue testing device of  claim 8  further comprising an additional fixture for fixing a second portion of the blade. 
     
     
       16. The fatigue testing device of  claim 15  wherein said additional fixture is adjustable to allow fixation at different locations along the blade. 
     
     
       17. The fatigue testing device of  claim 8  further comprising at least one optical sensor, said control system being further configured to modulate the excitation frequency based on a signal generated by said optical sensor. 
     
     
       18. The fatigue testing device of  claim 8  further comprising at least one laser displacement sensor, said control system being further configured to modulate the excitation frequency based on a signal generated by said laser displacement sensor. 
     
     
       19. The fatigue testing device of  claim 8  wherein said control system is further configured to modulate the excitation frequency so that the excitation frequency is continuously swept forwards above and backwards below the initial natural frequency of the blade. 
     
     
       20. The fatigue testing device of  claim 6  wherein said control system is configured to bring the blade under test up to an initial load at a frequency within 10 percent under the initial natural frequency. 
     
     
       21. The fatigue testing device of  claim 20  wherein the initial natural frequency is determined by a pre-test frequency sweep.